1. Field of The Invention
The present invention relates to color correction apparatus and method which are employed to correct color signals between the input means and output means of graphic data, and more particularly to variable point interpolation apparatus and method with scalable architecture for color correction.
2. Description of Prior Art
Nowadays, color scanning and printing means display different kinds of image capturing and displaying characteristics. This makes the nonlinear color difference between input means and output means is very apparent since each manufacturer differs in design and manufacturing procedures. Therefore, it is important in color image processing, exciting and extracting the color characteristics of the input and output means should produce a copy close to the original in chromaticity, to achieve satisfactory color reproduction.
In a process of color reproduction, color correction has to be made to produce an output having a chromaticity close to the original. There are many different methods of color correction in the prior art, such as "A Color Correction Scheme for Color Electronic Printers", written by G. K. Starkweather, Color Research and Application, vol. 11, pp. 367-372, supplement 1986; "Predictive Color Correction", written by Y. Sato and K. Imao, Proceedings of SPIE's Third International Congress on Advances in NIP Technologies, pp. 347-354, 1988; and "Color Hard Copy--a Self-Tuning Algorithm Based on a Colorimetric Model", written by B. Petschik, Printing Technologies for Images, Gray Scale, and Color, SPIE vol. 1, 1458, pp. 108-114, 1991. The above methods use polynomial regression analysis under a presumed mathematical model in accordance with the theory of masking method to make a finite color correction. However, it is difficult to execute color correction in a fixed mathematical model because of the non-linearity and high variety of pigments or dyes in different color printing processes. And the order number of polynomial expression is increased due to the high accuracy requirements of color correction. As a result, a lot of computation and time have to be spent in either hardware implementation or software simulation. Therefore, the above prior-art methods do not satisfy the requirements of electronic desk top publishing (EDTP) coming up in the branch.
On the other hand, in the paper entitled "Neural Network Applications to the Color Scanner and Printer Calibrations", written by H. R. Kang and P. G. Anderson, Journal of Electronic Imaging, vol. 1, pp. 125-135, April 1992, and also in U.S. Pat. No. 5,162,899, a method is disclosed which combines color input means (e.g., scanner) and color output means (e.g., printer) to form a closed loop for performing color correction. This method is an open type, i.e. an arbitrary combination of input means and output means can be made by the users to achieve the adaptive color correction. However, the efficiency is dependent on the accuracy of the input means as well as the resolution degree, metamerism, inverse halftoning, etc. And if the output of the output means is paper and the input means uses film and vice versa, this method will not work.
The direct color mapping method most often used to perform color correction of printing machines in traditional printing is generally performed by a skilled technician. Each of the colors is adjusted to an optimized color one by one before being printed. Hence, a delicate color reproduction can be made. This method, however, wastes too much time and memory space as well. For example, a 50-MB memory space is required for a 24-bit color image (each of R, G and B needs 8 bits). The process costs too much regardless of whether it is implemented by means of memory or a look-up table.
Therefore, to correct the drawbacks mentioned above, different interpolating methods are disclosed in U.S. Pat. Nos. 4,275,413, 4,334,240, 4,477,833, 4,511,989 and 5,241,373 to reduce memory requirements and maintain the accuracy of color reproduction. Now, most of color management systems adopt this kind of method to carry out color rendition. Although the interpolating method is widely adopted, it has some drawbacks such as (1) the interpolation architectures used in the prior art are fixed and unchangeable. Such architectures cannot be adjusted in response to the requirements of color patch and color characteristics, and therefore increase unnecessary interpolation computation and cause the production of secondary errors; (2) the prior-art interpolating methods adopt a fixed number of interpolation points so that the computation accuracy is too high at the portion where the human eye has low sensitivity for color difference and is too low at the portion where the human eye has high sensitivity for color difference; (3) conventional interpolation look-up tables are built by dividing the color space uniformly, which results in the fact that the contents of the LUT (Look-Up Table) are not accurate enough to produce color reproduction.